For a person to flush is to become markedly red in the face and often other areas of the skin, from various physiological conditions. Flushing is generally distinguished, despite a close physiological relation between them, from blushing, which is milder, generally restricted to the face or cheeks, and generally assumed to reflect embarrassment. Flushing is also a cardinal symptom of carcinoid syndrome – the syndrome that results from hormones (often serotonin or histamine) being secreted into systemic circulation.

Commonly referred to as the sex flush, vasocongestion (increased blood flow) of the skin can occur during all four phases of the human sexual response cycle. Studies show that the sex flush occurs in approximately 50-75% of females and 25% of males, yet not consistently. The sex flush tends to occur more often under warmer conditions and may not appear at all under coolertemperatures. It has also been commonly observed that the marked degree of the sex flush can predict the intensity of orgasm to follow.

During the female sex flush, pinkish spots develop under the breasts, then spread to the breasts, torso, face, hands, soles of the feet, and possibly over the entire body. Vasocongestion is also responsible for the darkening of the clitoris and the walls of the vagina during sexual arousal. During the male sex flush, the coloration of the skin develops less consistently than in the female, but typically starts with the epigastrium (upper abdomen), spreads across the chest, then continues to the neck, face, forehead, back, and sometimes, shoulders and forearms.

The sex flush typically disappears soon after orgasm occurs, but this may take up to two hours or so, and sometimes, intense sweating will occur simultaneously. The flush usually diminishes in reverse of which it appeared.